In this work, the validity of standard magnetocaloric (MCE) scenarios is revisited for the Hubbard model for a square (two-dimensional) lattice to describe a layered metal. Different types of magnetic ordering (ferrimagnetic, ferromagnetic, Néel and canted antiferromagnetic states) with magnetic transitions between them are considered to minimize the total free energy. The phase-separated states formed by such first-order transitions are also considered consistently. We employ the mean-field approximation to focus attention on the vicinity of a tricritical point, where the order of the magnetic phase transition changes from first to second and phase separation bounds merge. Two types of first-order magnetic transition can be found: PM-Fi, Fi-AFM; with further temperature growth, the phase separation boundaries between them merge and a second order transition, PM-AFM, is observed. The temperature and electron filling dependencies of the entropy change in the phase separation regions are investigated in detail in a consistent way. The dependence of the phase separation bounds on the magnetic field results in the existence of two different characteristic temperature scales. These temperature scales are indicated by giant kinks in the temperature dependence of the entropy, which are an exceptional attribute of phase separation in metals.

The electronic structure and optical properties of intermetallic compounds GdTi0.05MnxFe0.95 –xSi (x = 0, 0.4, 0.6, 0.95) have been investigated in this work. Spin-polarized densities of electronic states and optical conductivity spectra have been calculated by the DFT + U method, taking strong electron correlations in the 4f shell of Gd into account. The optical properties of these materials in the energy range 0.078–4.6 eV have been measured by the ellipsometric method. The nature of the quantized light absorption has been discussed, and a comparison of the experimental and theoretical spectra of the interband optical conductivity has been used. It has been shown that the change in the optical properties of the compounds when iron is substituted for manganese can be interpreted qualitatively based on the calculations of the densities of electronic states.

In this paper, the electronic structure and magnetic properties of the ternary intermetallics of the HoNiZ series (Z = Ga, Si, Al) is studied using the LSDA+U first-principles method. An analysis of the densities of electronic states in the compounds HoNiGa, HoNiSi, and HoNiAl with accounting for electronic correlations is performed.

 ФИЗИКО-МАТЕМАТИЧЕСКИЕ НАУКИ  УДК

The electronic structure and the exchange interactions in EuNi4Co and YbNi4Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi4Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.

AbstractThe electronic structure and the exchange interactions in EuNi_4Co and YbNi_4Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3 d lattice in both types of crystallographic positions 2 c and 3 g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3 g type. A Co ion in RNi_4Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3 d metal sublattice.